Ionization chamber



July 17, 1956 l J. J. KEYES, JR 2,755,391

' IoNIzATIoN CHAMBER Filed Dec. 18, 1952 ATTORNEY United States Patent OION IZATIDN CHAMBER John I. Keyes, Jr., Newark, Del., assigner to theUnited States of America as represented by the United States AtomicEnergy Commission Application December 18, 1952, Serial No. 326,629

2 Claims. (Cl. 25d-33.6)

The present invention relates to a gas ow ionization chamber forcontinuously monitoring a gas containing radioactive material.

Many laboratories today use radioactive isotopes for tracer analyses aswell as radioactive sources of high intensity for other experimentalwork. There are facilities known as hot laboratories, usually associatedwith nuclear reactors, which deal primarily with irradiated materials,many of which are highly radioactive. Depending upon the materials undertreatment, dangerous lission products, such as Xenonla, may be releasedinto the Ventilating air of these laboratories, together with activeparticulate matter. The atmospheric tolerance level for Xe133 is 2.0 l07microcuries per cubic centimeter. It is therefore essential that meansbe provided for detecting very low activities in the Ventilating air tosafeguard the working personnel from any accidental leaks of radioactivematter.

It is apparent that a very sensitive detector is necessary .tocontinuously monitor the Ventilating air in these laboratories. Theradioactivity inthe air is measured in apparatus embodying the presentinvention by passing the Ventilating air through the sensitive countingVolume so that even the lowest activities will be measured. Thedifficulty with this procedure is that the internal parts of theionization chamber, particularly the insulating portions, can becomequickly contaminated and thereby require frequent disassembling fordecontamination purposes.

A detector embodying the present invention prevents any such quickcontamination as well as preventing leakage of the radioactive gas. Ifat any time, after a long period of use, the surfaces of the detector dobecome contaminated, the chamber can be quickly dismantled fordecontamination purposes. The dismantling procedure is simplified by theabsence of any complicated seals in the ionization chamber.

More particularly, the present invention relates to a gas llowVionization chamber for continuously monitoring radioactive gas whichcomprises a rst electrode, a second electrode axially disposed withinthe rst electrode and insulated therefrom, means for establishing adifference of potential between said electrodes to form a sensitivecounting volume therebetween, shielding means enclosing said electrodesand spaced therefrom by electrically insulating spacers, means forintroducing and expelling the radioactive gas to be measured from saidsensitive counting volume, means for introducing a bufferingnonradioactive gas into the space between the electrodes and Saidshielding means, the buffering gas being at a higher pressure than theradioactive gas and means for measuring any current flow between the rstand second electrodes.

It is accordingly one of the objects of the present invention to providea new and improved lgas flow ionization chamber.

A second object of the invention is to provide a new 2,755,391 PatentedJuly 17, 1956 ice and improved quickly separable ionization chamber forcontinuously monitoring radioactiveV gas flow.

Another object is to provide an ionization chamber which protects theinsulating surfaces thereof by use of a non-radioactive gas.

Still another objectV is to provide an easily separable ionizationchamber which is inexpensive to fabricate and simple to maintain.

The many objects and advantages of the present invention may best beappreciated by reference to the accompanying drawing which is atransverse, sectional View of a preferred embodiment of the presentinvention shown in combination with diagrammatically representedconventional instruments.

Referring to the drawing, the apparatus is mounted within a stepped,cylindrical casing 10 having an upper annular ange 11 and a lowerannular llange 12. Connected to the upper ange 11 is a plate 13 whichseals the upper portion of casing 10 by means of a suitable gasket 14.Similarly, the lower portion of casing 10 is sealed by a base plate 16and a gasket 17 attached to flange 12.

Also mounted on the base plate 16 are insulated spacers i8 which supporta cylindrical chamber 20. Chamber 20 is preferably made of any suitableelectrically insulating material, such as Micarta, which is a laminatedphenol formaldehyde. The entire inner surface of chamber 20 is coatedwith an electrically conductive coating 21 such as Aquadag, which is agraphite coating.

Attached to the coatingZ at the upper end of chamber 2t) is contact 25connected to a conductor 22 which extends through the chamber and casingwalls and is insulated from casing 10 by a plug 23 made of anelectrically insulating fluorocarbon compound such as Fluorothene Aconductor 35 connects conductor 22 to one side of a microammeter 24, theother side of which is connected to the electrical ground. Theconnection of conductor 22 to conductor 35 is shown diagrammatically soas not to unnecessarily complicate the drawing, but it is apparent to aperson skilled in the art that this connection can be made in aconventional manner. For example, the conductor 35 may be removablyclipped to conductor 22.

Centrally located in the upper end of chamber 20 is a passageway 26through which extends a smaller diameter tubular member 30 made of anelectrically insulating material. Member 3d extends substantially theentire length of chamber 20 and has its inner and outer surfaces coatedwith an electrically conducting material 27, such as Aquadag Asindicated in the drawing, member 30 need only be coated up to a portionthat is external to the chamber Ztl. Attached to the coating 27 in thisportion is a second electrical contact di) connected to a conductor 28which extends up through plate 13 and is insulated therefrom by a plug29. A conductor 31 connects conductor 28 to the movable arm of apotentiometer 32 which in turn is attached to the output of aconventional direct current power supply 33.

Tubular mefrnber 30 is maintained in a coaxial position with respect tvchamber 2t) and casing 10 by means of electrically insulating spacers34. The inner surface 36 of the upper portion of casing 10 is preferablytapered,

s-,showm to properly support the tubing and spacer as rnbly. Holes areprovided in spacers 34 to permit -the passage therethrough of conductor23.

Also mounted on the upper plate 13 is an inlet conduit 37 for theradioactive gas to be monitored. Conduit 37 extends partially withintubular member 30. At the lower end, conduit 37 is connected to a rigidwire 38 n is V.axially disposed with respect to member 30 and extendsasubstantial length therein. f

Connected to the plate 13 and the stepped portion 39 of casing 10,respectively, are inlet conduits 41 and 42. Conduits 41 and 42 areconnected to a gas supply 43 by means of conduits 44 and 46,respectively.

Mounted on the surface of casing is an outlet conduit 47 for theradioactive gas. Conduit 47 extends through the surface of casing 10 andthrough an outlet passageway 48 in the surface of chamber 20 withouttouching the surface of chamber 20.

As indicated in the drawing, casing 10 is preferably made of anelectrically conductive material which can be conveniently grounded atits lower flange by means of a conductor 49. The casing 10 and plates 13and 16 therefore provide shielding means for the ionization chamberhaving cylindrical chamber as its cathode and tubular member as itsanode. Since the inlet conduit 37 is mounted on plate 13 and the wire 38is connected thereto, both conduit 37 and wire 38 are maintained atground potential.

In operation, the inlet conduit 37 and the outlet con duit 47 arepreferably mounted in series with the ventilating system to be monitoredor a representative portion thereof. Potentiometer 32 on the output ofpower supply 33 is varied so as to supply the proper direct currentpotential to the cylindrical anode 30 of the ionization chamber. Sincethe Aquadag coating 27 on the inner surface of anode 30 is continuousabout the lower end thereof, both the inner and outer surfaces of theanode are maintained at the desired D. C. potential.

As the radioactive gas ows through inlet conduit 37 down through anode30, any static charge present in the gas will be removed by the groundedwire 38. As the gas ows upwards from the bottom of anode 30 to theoutlet conduit 47 near the top of chamber 20, any ionization occurringin this volume due to the presence of a radioactive material will causeions to impinge on the inner surface of cathode 20, that is, thepositive ions will be repelled by the high positive D. C. potentialpresent at the outer surface of anode 30. The potential of the coating21 will be raised in proportion to the number of impinging ions. Thischange of potential is measured by meter 24. It is apparent that meter24 can be connected to a conventional recording device if it is desiredto obtain a permanent record of radioactive intensity of the gas in theVentilating system. Similarly, conventional alarm limits may beincorporated in meter 24 to operate when the reading corresponds to anintensity level equal to the tolerance level of 2.() 1OI mierocuries/cc.or less if desired.

The grounded casing 10 serves as a guard ring between the cathode 20 andthe anode 30 so that leakage across the insulating spacers will not berecorded on the meter. As can be seen in a demountable apparatus of thistype, the insulating surface could easily become quickly contaminatedthereby providing a leakage path for stray currents so that the accuracyof the meter will become seriously affected.

To prevent this, a buffering non-radioactive gas is introduced from gassupply 43 through conduits 41 and 42 into the spaces between each of theelectrodes and the casing 10. This buffering gas is preferably dry airor nitrogen and is maintained at a slightly higher pressure than that ofthe introduced radioactive gas. Since the outer diameter of conduit 37is smaller than the inner diameter of anode 30, and the outer diameterof the anode 30 is in turn smaller than the inlet passageway 26 ofchamber 20, none of the incoming radioactive gas will flow into thespaces between the casing 10 and the anode 30. Similarly, since theouter diameter of outlet conduit 47 is smaller than the diameter ofpassageway 48, none of the radioactive gas flowing out of the chamber 20will flow into the space between casing 10 and chamber 20. That is, thenon-radioactive gas acts as a buffer between the chambers insulatingsurfaces and the radioactive gas.

Therefore, the chamber can be operated continuously for long timeintervals without contaminating the guard ring and thereby providing aleakage path to upset the accuracy of the meter.

If it is necessary to repair or decontaminate the apparatus for anyreason, the anode 30 may easily be removed by removing the plate 13.Similarly, the cathode 20 can be removed by removing the base plate 16.It can be seen that the dimensions are such that casing 20 could beslightly tilted to permit passageway 48 in cathode 2lb to become free ofconduit 47.

lf the chamber is used in a Ventilating system for aI hot laboratory, itis preferable that the chamber be made in a comparatively large size soas not to impedel the flow of Ventilating air. One embodiment whichoperated satisfactorily used a cathode with a 10-inch internal? diameterand a length of inches. The diameter of the anode was 1% inches, whereasthe overall lengthY of the apparatus was approximately inches. Thisapparatus used a D. C. potential of 250 voltsV and also used a BeckmanRXG2 micro-microammeter. This chamber gave a response of 5x10-14 amperesfor 10-13 curie/cc; of Xe133.

Since many embodiments of the present invention might be made and sincemany other changes may be made in the preferred embodiment describedabove, it is understood that the foregoing description is to beinterpreted as illustrative only and not in a limiting sense except asrequired by the appended claims.

I claim:

l. A separable ionization chamber for continuously monitoring aradioactive gas which comprises in combination, an electricallyconductive base plate, a cylindrical chamber mounted in insulatedrelation thereon and having a passageway in one end thereof, a tubularmember mounted coaxially with said chamber and extending through saidpassageway substantially the length of said chamber, the outer diameterof said tubular member being smaller than the diameter of saidpassageway, a stepped, cylindrical, grounded casing mounted on said baseplate and completely enclosing said chamber and tubular member, an inletconduit for said radioactive gas mounted on said casing and coaxiallyextending within one end of said tubular member, the outer diameter ofsaid conduit being smaller than the inner diameter of said tubularmember, an outlet conduit for said radioactive gas mounted on Saidcasing surface and extending through a second passageway in the surfaceof said chamber, the outer diameter of said conduit being smaller thanthe diameterof said second passageway, means for introducing a bufferingnon-radioactive gas into the space between said casing and said chamberand tubular member, said buffering gas being at a higher pressure thansaid radioactive gas, means for establishing a difference of potentialbetween said tubular member and the inner surface of said chamber, andmeans for measuring the number of ions reaching the inner surface ofsaid chamber.

2. Apparatus as recited in claim l in combination with means forremoving a static charge in the introduced radioactive gas.

References Cited in the tile of this patent UNITED STATES PATENTS2,423,411 Simpson July 1, 1947 2,440,475 Jacomini Apr. 27, 19482,489,925 Ornwake Nov. 29, 1949 2,532,613 Dutcher Dec. 5, 1950 2,532,874Anderson Dec. 5, 1950 2,622,208 Bernstein et al. Dec, 16, 1952 2,625,657Kanne Jan. 13, 1953

1. A SEPARABLE IONIZATION CHAMBER FOR CONTINUOUSLY MONITORING ARADIOACTIVE GAS WHICH COMPRISES IN COMBINATION, AN ELECTRICALLYCONDUCTIVE BASE PLATE, A CYLINDRICAL CHAMBER MOUNTED IN INSULATEDRELATION THEREON AND HAVING A PASSAGEWAY IN ONE END THEREOF, A TUBULARMEMBER MOUNTED COAXIALLY WITH SAID CHAMBER AND EXTENDING THROUGH SAIDPASSAGEWAY SUBSTANTIALLY THE LENGTH OF SAID CHAMBER, THE OUTER DIAMETEROF SAID TUBULAR MEMBER BEING SMALLER THAN THE DIAMETER OF SAIDPASSAGEWAY, A STEPPED, CYLINDRICAL, GROUNDED CASING MOUNTED ON SAID BASEPLATE AND COMPLETELY ENCLOSING SAID CHAMBER AND TUBULAR MEMBER, AN INLETCONDUIT FOR SAID RADIOACTIVE GAS MOUNTED ON SAID CASING AND COAXIALLYEXTENDING WITHIN ONE END OF SAID TUBULAR MEMBER, THE OUTER DIAMETER OFSAID CONDUIT BEING SMALLER THAN THE INNER DIAMETER OF SIAD TUBULARMEMBER, AND OUTLET CONDUIT FOR SAID RADIOACTIVE GAS MOUNTED ON SAIDCASING SURFACE AND EXTENDING THROUGH A SECOND PASSAGEWAY IN THE SURFACEOF SAID CHAMBER, THE OUTER DIAMETER OF SAID CONDUIT BEING SMALLER THANTHE DIAMETER OF SAID SECOND PASSAGEWAY, MEANS FOR INTRODUCING ABUFFERING NON-RADIOACTIVE GAS INTO THE SPACE BETWEEN SAID CASING ANDSAID CHAMBER AND TUBULAR MEMBER, SAID BUFFERING GAS BEING AT A HIGHERPRESSURE THAN SAID RADIOACTIVE GAS, MEANS FOR ESTABLISHING A DIFFERENCEOF POTENTIAL BETWEEN SAID TUBULAR MEMBER AND THE INNER SURFACE OF SAIDCHAMBER, AND MEANS FOR MEASURING THE NUMBER OF IONS REACHING THE INNERSURFACE OF SAID CHAMBER.